1. Technical Field
The present invention relates to a light source device, an illumination system, and a projector.
2. Related Art
A projector is known as an apparatus capable of displaying a large-screen image. Atypical projector includes an illumination system, an image forming device, and a projection lens, for example. Illumination light emitted from the illumination system forms an image by the function of the image forming device. The image is expanded and projected by the projection lens such that a large-screen image can be more easily produced than a direct viewing type image display apparatus.
A known light source device included in the illumination system has a light source such as a high-pressure mercury lamp, and a reflector. The reflector has a shape of a paraboloid of revolution or spheroid whose focus is located at the position of the light source. The reflector used in related-art is a full-surrounding type which has a reflection surface surrounding the entire circumference of the light source around the revolution axis containing a light emission point. According to this structure, light emitted from the light source is reflected by the reflector and extracted to the outside of the light source device.
There are several points currently expected for the projector, including size reduction of the apparatus, lower power consumption, and high-luminance image display. Japanese Patent Nos. 3350003 and 3923560 propose technologies for reducing the size of the projector.
According to Japanese Patent Nos. 3350003 and 3923560, a light source device includes a first reflector, a second reflector, and a light source. The reflection surface of the first reflector is constituted by a part of a spheroid as a division of the spheroid cut along division surfaces containing the revolution axis. The reflection surface of the second reflector is a part of a sphere as a division of the sphere cut along the same division surfaces. The reflection surface of the first reflector is disposed to oppose to the reflection surface of the second reflector. One focus of the reflection surface of the first reflector coincides with a focus of the second reflector and a light emission point of the light source.
Light emitted from the light source toward the second reflector is reflected and bent by the second reflector, and then passes the light emission point and enters the first reflector. Light emitted from the light source toward the first reflector travels to the outside of the light source device together with the light having passed the second reflector.
According to the technologies shown in Japanese Patent Nos. 3350003 and 3923560, the second reflector has substantially the same size as that of the light source. Thus, the size of the light source device can be decreased to approximately half. Moreover, a projection area of the reflection surface of the first reflector projected on a plane perpendicular to the optical axis is approximately half of that area of the full-surrounding type reflector. Thus, the spot size of the light extracted from the light source device becomes approximately half. Accordingly, the sizes of optical elements such as an image forming device, a projection lens, and lenses receiving light from the light source device can be reduced.
While the projector shown in Japanese Patent Nos. 3350003 and 3923560 is considerably compact, the following points need to be improved in view of reduction of light loss.
It is known that a discharge-type light source emits light based on a light emission theory described herein. Discharge gas sealed into an arc tube (discharge tube) collides with electrons produced by arc discharge. As a result, the discharge gas is partially excited, and is partially ionized to become plasma. The exited discharge gas returns to ground state or metastable state, thereby emitting light corresponding to the energy difference between the exited state and the returned state. The discharge gas as plasma recombines with electrons and emits light corresponding to combination energy.
According to the light source device shown in Japanese Patent Nos. 3350003 and 3923560, the luminance distribution of the light reflected by the second reflector almost agrees with the luminance distribution of the light source image with respect to the pattern and position. That is, a higher luminance portion of the light reflected by the second reflector enters a higher plasma density area. In this case, light loss becomes larger than that of the light source device having the full-surrounding type reflector, and therefore problems such as difficulty in displaying high-luminance images and higher power consumption of the projector arise.